Micca Bookshelves or Pioneer SP-BS22-LR... the latter is currently 89.99 if you have prime and is about the best you are going to get sub $100/pair. Ignore a certain poster who feels everything sub $XXXX.XX dollar value "isn't worth your time." He's just grumpy he's been banned from major audio forums across the web so he spews his bile here.

Check out Bic America bookshelf DV62si. You can pick them up much less than MSRP and they sound great for the price. I set up a 7.1 surround for a friend on a tight budget using two sets of those with three centers across the front and was very pleasantly surprised with the quality of sound for the price.